Transverse Section of Rhondda Valley.
The Rhondda river unites with the Taff river twelve miles below Quaker's Yard, and fifteen miles from its source.
The transverse section across C measures 1730 yards from the opening of the levels on the opposite sides, 84 yards on the west side, and 79 yards on the east side, above the level of the Rhondda river, at the point C.
The gradient from the west level (D) to the river is 1 in 13, that from the east level (C) is still steeper, being 1 in 812; so that the sides of the valley slope at 1 in 11 on the average (from C and D to the river), or at an angle of 8°. The point C is about four miles from the furthest watershed of the river beyond.
Below the level of the quarry, on the west side, above the river, there is no rock seen except at one point in a watercourse.
The dip of the Coal-measures at this point (D E) is about 3° north, and the section (Pl. IV. fig. 3, D E) is nearly east and west.
The escarpment of the high ground above D and E is nearly twice as steep and high as that of the South Downs, and pursues its course nearly north and south along each side of the Rhondda valley. The dip varies, sometimes being northerly, and at other times south. This is marked by the arrows in the Ordnance map; and the lines of outcrop of the coal also show the direction of the escarpment. The escarpment, therefore, in the Rhondda valley is in the direction of the dip, and not of the strike; and the dip changes without any alteration in the direction of the escarpment. In fact, in this district, we have in a short distance escarpments parallel with the dip and strike, and also at almost every angle to them.
The valley is at D E about 300 yards deep, but is not shown above the quarry in the diagram, as I do not allude at the present time to anything more than the portion of the gravel in the valley below the No. 2 vein of coal. I visited this pit several times during the progress of the sinking, and counted the proportion of rolled to weathered stones each time. About one-eighth of the gravel in this pit was well rolled, the remainder being generally weathered.
It is evident that if only one-eighth of the gravel is rolled by the river, seven-eighths of it must have been obtained from the sides of the valley, west and east of the pit, and only one-eighth from the north, and that the valley was then widening much faster than it was lengthening in the latter part of the Quaternary period. There are times in the denudation of valleys when pluvial action on the sides of the valley is much greater than fluvial action in the centre of the valley. I mentioned (Quart. Journ. Geol. Soc. vol. xxiv. p. 117) that even in the valley of the Somme, where the fall of the river is so small, a great mass of the material deposited in the concavities of the valley near the margin of the river is derived from the adjacent country, and has never been river-borne. Different as is the position of the gravel of the Rhondda, in this respect it resembles that of the Somme.
The gravel-deposit continues for the whole length of, and is also met with at a considerable height above the Rhondda river. The
